In the third trimester, there is no need to hold urine for examination, as the enlarged uterus has grown out of the pelvic cavity and the fetus, amniotic fluid and placenta can be directly observed through abdominal ultrasound. In the third trimester, since the uterus is still in the pelvic cavity and the bladder is blocked in front of the uterus, it is necessary to fill up the bladder during the abdominal ultrasound so that the development of the embryo in the uterus can be observed by using the bladder fluoroscopy window. In rare cases, women with irregular menstrual cycles, menstrual periods that come once every two months or small fetal development, where the uterus does not increase in size and grow out of the pelvic cavity in the third trimester of pregnancy, also need to hold urine for examination.